Thinking back upon my earliest radio astronomy experiments, I
remember clearly the excitement of seeing those first charts.
After a false start or two with bad connectors and circuit
issues, I began seeing stripcharts that really looked like
they contained a lot of fascinating objects. I was working at
612 MHz and my antenna was the 12 foot stressed cylindrical
paraboloid that some of you may have seen the Radio Astronomy
Teacher's Notebook. It took a while for me to figure out
that most of the curious bumps on the chart could not be
the signatures of celestial objects passing through my antenna
beam. Understanding a few basic concepts would have helped me
sort out which bumps were real objects and which ones were
anomalous.

As the World Turns...

Drift scanning is the method most of us use in amateur radio
astronomy. We point our antenna along the Southern or Northern
meridian, an imaginary line drawn from one of the celestial
poles to an azimuth on our horizon at 180 or 0 degrees
respectively. Every celestial object must pass through the
meridian at some point during the day (if the object is visible
from your latitude at all).

Intuitively, we can readily see that objects close to the
celestial pole arc through a smaller circle as the Earth spins than
do those close to the celestial equator. If the object were at the
celestial pole, it would seem not to move at all. It makes sense
then that the declination of the object has a lot to do with how
fast it appears to travel through your antenna beam.

At the celestial equator ( declination = 0 ) objects appear to
move at a rate of about .25 degrees per minute. To get the rate
at any other declination multiply .25 * cosine of the declination.
For example, at declination 40.7 (Cygnus A's declination) we get:

.25 * cosine(40.7) = 0.19 degrees per minute

Cassiopeia A is at declination 58.7 degrees so it appears to move
at:

.25 * cosine(58.7) = .13 degrees per minute.

The Beamwidth Factor

Our antennas are sensitive to signals arising from more than a
single point in the sky. They are sensitive to a region which
has an angular width. We usually define this region's width by
the angular extent of the beam that produces a power level in
our receivers of at least one half of that produced at the
point of maximum sensitivity at the beam's center. This is the half
power beamwidth (or HPBW ). That is a handy definition because
it gives us a clue as to what measurement we need to help
identify a source which passes through the beam.

In order to continue with the investigation, you have to know
your antenna's HPBW. If you know your antenna's gain (G) as
compared to an isotropic antenna, you can calculate the HPBW:

HPBW = SQR(41253/G) [SQR indicates Square Root]

Important Note: Here the gain G is expressed as a simple power
ratio. Sometimes G is expressed in dB over the gain of an
isotropic antenna (GdBi). If you only know your antenna gain
in dBi then first convert it to the simple ratio
gain used here [ G = 10^(GdBi/10) ]. Also, if the specification
you have at hand for your antenna's gain is then gain over a
dipole antenna in decibels (GdBd), add 2.14 dB before
applying this formula [ G = 10^((GdBd+2.14)/10) ].

This gain to HPBW formula also assumes a beam that is approximatly
circular. Some beams are not. For this purpose, we are interested
in the HPBW in the East-West direction.

Lets say we have an antenna G of 15, then:

HPBW = SQR(41253/15) = 52.4 degrees

The Big Deduction

Now we can make a calculation of how quickly the fastest moving
objects (those at declination 0) can make it through our beam.

HPBW/.25 = half power beam transit time in minutes.

So for our HPBW of 52.4 deg we get 210 minutes or about 3 and
a half hours! We should be very suspicious of anything that
we see on our chart which shows half power points occuring any
faster than this rate.

What if the object is at a different declination? We just bring
back the old cosine function, apply it to the declination and
multiply it by .25. Then we can divide the HPBW by that value.
For Cygnus A and the same antenna:

HPBW/(.25 * cosine(40.7)) = HPBW/.19 = 275 minutes.

We can deduce, therefore, that an object on the chart that
displays half power points AROUND 275 minutes at the right
time of day has a chance of being Cygnus A.

Other Tests

Before jumping to the conclusion that you have actually recorded
a given object, be sure you can repeat the observation many times.
Over a period of months you should see that the object
appears about 4 minutes earlier each day because of sidereal
motion. If the chart signature rises or falls very quickly, you
should be suspicious. Our beam patterns do not have sharp edges
to them!

This article describes a method for use with a simple total power
type telescope. With an interferometer, you have more clues as to
whether or not a chart signature represents a real cosmic source.
We will cover that in a future article.

I know how exciting it can be to see first light on a new radio-
telescope, but having an idea of what to expect in the way of
results can really help make observing more enjoyable and may
prevent you from making an embarrassing claim that someone else
will be sure to point out.

____________________________________________________________________

Windows Xp - Should I use it for Radio Astronomy?

Writing software for general distribution on Windows operating
systems can be frustrating to say the least. As soon as you iron the
issues on one OS, along comes another Microsoft money maker to send
you back to the keyboard. So, I was less than thrilled when Xp was
announced for release. Reluctantly, I went out and bought a copy of
Xp Home Edition to test my software on this new platform. As usual,
there is some good news and some bad news.

Xp loaded onto my multi-operating system PC with no problems. That was
a pleasant surprise. It did a nice job of detecting hardware and
network connections. At last Xp shows it's pretty face. Actually, it
is more cutesy than pretty in Fisher-Price sort of way. But I wanted
to investigate how well Xp would run my software and what others
could expect who want to use Xp on their computers to control their
own custom interfaces to their equipment.

What I found was that this OS is apparently quite stable and it offers
the computer user some tools for investigating problems which might
occur. In this respect, Xp is much more like Microsoft's Windows NT.
This is good news for people who need to run long term observations in
unattended mode. Chances are, the OS won't crash in this situation
the way Win95 and its derivatives Win98/Me are prone to doing.

To help with integrating in software not written especially for Xp,
you can right click on the program icon and select properties. From
there you can select to run the program in various modes, Win95, NT,
etc.. I am not sure whether this really helps solve compatibility
problems, but you might give it a try if things don't go well with
your old software. I had heard a rather vicious rumor that you cannot
run old DOS software on Xp. I had no problem with running Radio-Sky
Planetarium 1.2, a DOS program. The same success was experienced with
my 32 bit Win95/98 programs. I was quite happy until....

Many amateur experimenters use their computers to control custom
devices like ADCs, antenna rotators, and attenuators. The easiest
route from the computer to outside devices for experimenters is
probably the printer port. The printer port allows you to send digital
signals in and out of the computer over several pins on the 25 pin
connector without having to translate into RS232 or some more obscure
format like that of the USB port. To access the printer port, the CPU
must send an In or Out command to the right address. Unfortunately,
Microsoft has decided that this powerful capability is to much
responsibility to place in the hands of us mortal users. If you try
to submit a direct command to the processor to perform an In or Out
function in Xp or Windows 2000, you will meet with either an "Illegal
Operation" screen or your command will simply be ignored.

If you have your own custom or commercial I/O card, or you have
projects which rely on your software commands to the printer port,
you will have to either obtain new drivers or write your own. Writing
your own drivers will probably be impossible for DOS programs. If
you are using DOS and I/O you are out of luck. If you write in a
language that supports Active X controls you can obtain rather
expensive Active X components that will allow you to regain port
control. Presently, I am investigating Active X controls to regain
ADC capability in Radio-SkyPipe and a fully Xp compatible release
is expected soon. Writing drivers from scratch is a very complex
endeavor. The time and learning curve costs are probably too steep
for the average amateur to endure.

If you are thinking about moving to Xp (or Win2000), do so with
open eyes!

___________________________________________________________________

Radio-Sky News

Radio-SkyPipe Version 1.1.30 Available

It seems like every edition of the Radio-Sky Journal has one of
of these update notices. That is because the program just keeps
getting better and better. This new edition has several bug
fixes, and new features. The program is more reliable and can
now operate in a wider range of conditions including better
support to work through firewalls. If you are already a Radio-
SkyPipe user of version 1.1.24 or higher, you can download an
update patch that is a considerably smaller download than full
install.

The MAX186 analog to digital converter, for use with Radio-SkyPipe
or your own custom software will soon be available. The kit will
include a printed circuit board, MAX186 ADC IC, all on-board
components, and a layout template. No connectors are supplied for
the computer printer port or individual input ports in the kit.
The MAX186 KIT will be priced at $45 US plus shipping.

Also, soon available, will be completed MAX186 ADC boxes for $115
US plus shipping. These units will be in a compact metal enclosure
with a DB25 pin connector for attachment to the parallel port and
individual RCA type phone jacks for connecting to the eight input
channels.

A small 9-12VDC wall-wart type power supply will serve
adequately to power either unit (not supplied).

We have also been selling MAX186 ICs for some time now on the
website. We have a good price of $18.00 US plus shipping for these
ICs.

............................................................

Radio Jupiter Pro II

I am just about ready to release Radio Jupiter Pro II. This
Jupiter noise storm prediction program has many new features to
help you schedule your Jupiter observations. With new automation
features, you can do things like start a Radio-SkyPipe (or your
own data collection program), or turn your RS232 driven antenna
rotators to track Jupiter, all based on storm predictions and
Jupiter position. A 30 day evaluation download will be available
and registration for the program will only be $19.95. Keep an
eye our website front page for this.
_________________________________________________________________

The European Radio Astronomy Club

A thriving group for amateur radio astronomers has been established
in Europe. The European Amateur Radio Astronomy Club (ERAC) was
born in 1995, largely through the efforts of Peter Wright in
Mannheim, Germany. The ERAC already has about 250 members in
16 countries. They produce a 150 page paper journal 4 times a
year, and host a International Congress for Radio Astronomy every
three years. The next session of the Congress is in September of
2003.

You may contact the ERAC by email at: erac@wegalink.com

Their website is: http://www.erac.wegalink.com/

They also host a mailing list which can be subscribed to by sending
an email with no subject or contents to: erac-subscribe@egroups.com

___________________________________________________________________

Amateur Tip #7

I have never seen this published, anywhere that I can remember, but
this trick has worked for me to quite GAsFET LNAs that I have built
for the HI band (1420 MHz). Sometimes the box that you build your
LNA in can act as a cavity and induce oscillations in these sensitive
high gain devices. It is best to use as small a box as is practical,
but you can also try lining the box with the antistatic foam that ICs
are sometimes shipped on. In my experience, filling the available space
around the circuit elements, (without touching them of course), with
the anti-static foam seems to prevent some oscillation problems. It
is interesting to note that at the NRAO, LNAs are built by machining
narrow channels in solid blocks of metal and using special components
which closely match these channels, thus almost completely removing
the chamber around the circuit.

Amateur Tip #8

Radio Shack sells a 50 ohm BNC termination for LAN use that works well
for a low power RF applications up to at least 1 GHz. The part number
is 278-270. If you have a power splitter in your circuit, be sure to
terminate unused outputs in the proper impedance termination. Another
use for a termination (or "dummy load" as it is sometimes called) is
as a substitute for your antenna during initial tests of your system.
Take the time to record a couple of days of charts using the
termination instead of the antenna to reveal drifts and instabilities
in your system that might otherwise be mistaken for celestial sources.

___________________________________________________________________ Featured Sites

Take a look at the 408 Mhz All Sky Survey website. I really enjoyed
seeing how this dedicated group of amateurs was putting together a
viable project on reasonable budget. The rural setting in Britain
makes it all the better.

Again from Europe, if you haven't visited this great Belgian site
in a while, go back and see what Jean Polard has done with the
Merlin Observatory. There are some schematics of how he has attached
to his Sat Finder for the 4 and 10 GHz observatory.